ClientData

ClientData is sent from the FIDO Client to the relying party, and its hash will be sent
back from the relying party to the Client for verifying the signature on both registration
and authentication responses.

The registration and authentication request messages contain a challenge parameter, which
is defined as the SHA-256 hash of a (UTF8 representation of a) stringified JSON data
structure that the FIDO client has to prepare. The FIDO Client must send the Client Data to
the relying party during the verification phase, where the relying party can regenerate the
challenge parameter (by hashing the client data), which is necessary in order to verify the
signature both on the registration response message and authentication response message.